Track Testing Pull Bars on a Mustang Super Late Model

Be sure to check out the video highlights of our test at the end of the story!

Testing is extremely important. Anytime we conduct a test, be it at the track or on a dyno, two things come to mind. One is a tech column Bob Bolles wrote a while ago about all of us being scientists. Testing in an effort to find a result (favorable or unfavorable) yields useful information that we use, regardless of if it’s positive or negative. The other is something I heard at AETC, which is “you either have data, or you have an opinion.” Much of our testing starts from an opinion or a logical theory, then through testing the desired outcome is either validated or disproved, which is how this story came to be.

Bob wanted to test a pull bar on Dick Anderson’s Mustang Super Late Model to see how rear steer would affect the car exiting the corners. The original theory was that letting the rear differential rotate under acceleration when the trailing arms are mounted at different heights would create rear steer to the left, allowing the car to have more bite off the corner. Another benefit of this should be a much more stable feeling for the driver, as the rear of the car should stay square instead of drifting towards the wall exiting the corner.

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To test this, we headed to Desoto Super Speedway in Bradenton, FL on two separate occasions to test two different pull bars—one formally sold by Allstar Performance and one currently availablefrom Coleman Racing Products. The Desoto 1/3-mile oval sports 14- to 16-degrees of banking and is a pretty fast and rough track. In our last test with Dick, the car was run at Orlando Speed World, which is a much flatter track (about 6 degrees). The setup for the flatter track will make the car tight on the higher banked surface. To alleviate this, the Panhard bar was raised to make the car neutral.

After some baseline laps, the solid third link came out and the Allstar Performance pull bar went in. This bar has a 1,600-pound spring and an internal hydraulic piston with a remote canister to adjust the preload of the spring. The bar was installed with the adjustment set to full loose. Our Replay XD camera was pointed at the bar when Dick headed back out on track, and the pull bar instantly went to work. The bar moved about 3/4-inch and let the rearend rotate about 4 degrees, which equates to an 1/8-inch of rear steer.

6/16With the baseline complete, the solid third link was swapped for the adjustable unit from Allstar Performance. It has a 1,600-pound spring inside the canister and spring preload is adjustable with the remote mounted canister.

10/168 Back out on track, Dick’s times dropped 2-3 tenths almost instantly. The car felt more stable and Dick was able to drive harder off the corners, just as Bob had predicted.

“The car felt pretty good with the solid link, “ explained Dick Anderson. “We put the Allstar pull bar on the car and I could feel an improvement. It snugged the car up, which it needed at that track.”

After analyzing some of the video from the day of testing, we wanted to make a few changes to see how the car would react, and we started with the bump springs. The car runs 1,200-pound bump springs. In an attempt to see what they are doing on track, we put one of our Replay XD cameras under the hood pointed at the base of the shock. We were surprised at what we found. The 1,200-pound spring came very close to coil-bind. The issue was remedied by swapping the 1,200 pounders for 1,500-pound springs, which reduced the amount of travel in the bump spring. Travel is camber change and we want to limit that while still providing compliance (movement of the suspension over the bumps). Dick didn’t notice the difference between the two on the track, but less travel without being close to coil-bind is a more favorable setup. A second video verified the change worked, and you can see both videos at www.circletrack.com.

From there, the Allstar pull bar was pulled in favor of the simpler unit from Coleman Racing Products. This pull bar uses a 1,000-pound spring, and should let the rearend rotate farther than the 1,600-pound Allstar bar. The video confirmed that the lighter-weight spring allowed the rear to rotate almost twice the distance. With the rearend rotating 1.4-inches and about 73/4 degrees, rear steer increased to just under a 1/4-inch.

“I could really feel the reared moving with the Coleman pull bar,” explained Dick. “If I turned a perfect lap, the car felt great, but if I was on and off the throttle, it was a handful. The pull bar has a lot of potential, we just need more time testing with is.”

14/16The Coleman pull bar has a really nice travel indicator, which lets us know exactly how far the rear has rotated.

Dick told us the car was tight in the middle and not good going into Turn 3. We found two things causing this. First, the sway bar was loading way too much when the car dropped down on the bumps. This was corrected by backing the bar off two and a half turns. Second, the left front shock had too much rebound. After taking two clicks of rebound out of it, Dick said the car felt much better and drove well through the turns.

At this point we took a good look at the data we had collected thus far. When looking at the video from both pull bars, the pull bar slammed closed as soon as Dick lifted off the gas. The Penske shock installed to dampen the pull bar’s movement did not have enough compression resistence to control the rotation of the rearend. With the movement being so abrupt, it leaves the chassis feeling unsettled, and when you are on and off the throttle, this feeling is amplified.

A stiffer shock will keep the rearend stable while it’s in transition, as it will slow the movement of the rear end. The initial movement of the rear rotating backwards doesn’t need to be slowed, but when you lift off the throttle and the rear slams back into its normal position, it rotates so quickly and abruptly, that it actually bounces and upsets the car. A shock that has stronger compression, equal to the spring rate, will slow this movement, calming the rearend to settle back to its normal position easier.

15/16The car reacted well to the Coleman pull bar. The car became a little unpredictable in situation where Dick had to work the throttle, which is due to how far the rearend is rotating. We will try a 1,400-pound spring to limit the movement, along with a heavier dampener to slow the rearend’s rotation.

Something else that needs to be addressed with any of these pull bars is proper driveline geometry. Whenever the rearend rotates, the pinion angle changes. In our case, an inch and a half of rearend rotation equals 7 degrees of pinion angle. The static driveline angles need to be set with rearend rotation in mind. This is to be sure that under acceleration there are equal and opposite angles. We also found that a shorter third link would give us the pinion angle we need with the rearend rotating. In the future, we are also going to try a 1,400-pound pull bar spring, as an inch and half is probably more movement than we need.

16/16Over the course of both test days, Bob Bolles took exact notes and everything that happened. This data helps us come to a solid conclusion of exactly how everything worked.

Our test took place over two days, which were almost a month apart. Because of this, we haven’t really talked about lap times and time gains on track. But without seeing gains on the stopwatch, we saw gains on the stopwatch (this doesn’t necessarily make sense, but I’ll explain). When we made the change from the solid third link to the Allstar pull bar, we went 2 tenths quicker right away. We also made some setup changes as each test went on. But over the course of each test, which consisted of about 75 laps, our lap times were consistent and never fell off. The first test was on a set of old tires and the second test was on a set of new tires. Each test ended with lap times on par with our early lap times. Looking at this data, we strongly feel you would see a solid 3 or 4 tenth gain if each set of laps were done on equal tires. The other benefit we found is that this setup should lend itself well to long races, as the car remains very drivable over long on-track periods.

Be sure to go to www.circletrack.com to check out the ReplayXD video from our tests! And feel free to email us with your questions, comments, or experiences.